Some Prince William County supervisors, who reluctantly raised the real estate tax rate last week, expressed anger yesterday at the disclosure by the school superintendent of a $1.7 million surplus in the school budget.

"It's unethical," said Republican Supervisor Guy Anthony Guiffre in an interview. "This is taxpayers' money. It doesn't belong to the School Board."

Some members of the Board of Supervisors threatened to withhold money from the schools next year and roll back at least part of the 3-cents-per-$100 increase in the real estate tax rate that was adopted April 23.

School Superintendent Richard W. Johnson, who disclosed the surplus after the tax rate was raised, could not be reached for comment yesterday. Other top school officials said they had known for the past month or so that the county's schools were running a surplus of more than $1 million.

The $1.7 million surplus, which results from a combination of employe turnover, tight spending controls and a mild winter that produced unexpected fuel savings, is not all that unusual, these officials said. They said it represents a small fraction of the school system's $131 million operating budget.

"I saw no reason" to tell the county board about the surplus earlier, said school Finance Director John G. Colson. "We've never done that."

Colson added that in the past the county supervisors have instructed the schools to keep any surplus funds to use as the School Board sees fit.

Prince William County, with almost 36,000 students, has the third largest school system in Virginia. A third of Prince William's population is school-aged, and the county spends 54 cents of every tax dollar on education.

Guiffre and some other county supervisors say that if they had known about the school surplus before adopting the fiscal 1986 county budget they might have been able to avoid, or at least trim, the unpopular increase of the real estate tax rate to $1.45 per $100 of assessed value.

"We go through a long budget process and then we find out the day after our final conclusion that we didn't have all the information we should have," said Supervisor Kathleen K. Seefeldt, a Democrat.

Seefeldt said that she has asked County Attorney John H. Foote to look into the board's legal options in the situation, including setting a new tax rate and withholding appropriations to the schools next year.

About $70 million of next year's $146 million school budget comes from the county; the rest is drawn from state and federal sources.

According to Foote, the county cannot take money away from the schools when it has been appropriated for the year, but it is not legally bound to hand over all the money called for in the fiscal '86 budget.

Not all of the supervisors found the surplus unusual.

Board chairman G. Richard Pfitzner, a Democrat, called the timing of Johnson's disclosure "pretty tacky," but added: "I don't have any problem with it."

Pfitzner said that he will oppose any effort to change the tax rate set last week or to withhold money due the schools next year.